The classical buck-boost control uses only one PWM control signal and employs only 1 operation mode. It suffers poor conversion efficiency due to its high switch driving loss and high inductor current requirement.
A recent buck-boost converter employs different operation modes, and use more than one PWM control signal. Patent (U.S. Pat. No. 6,166,527 to Dwelley et al.) describes 3 mode operations comprising a buck, a buck-boost and a boost mode. This improves the efficiency, but it still has efficiency issue in the buck-boost mode. In this mode, all four switches have to work, and driving loss of switches is still high, and reduces the efficiency at light load condition.
Another recent buck-boost converter (US Patent Publication 2012/0146594 A1 to Kobayashi) employs 5 operation modes, which have a buck, a half-frequency buck, a half-frequency buck-boost, a half-frequency-boost, and a boost mode, and has ability to reduce the switching frequency in half. This improves the efficiency when input voltage is close to the output voltage while it maintains regulation performance.
These buck-boost converters using different operation modes might have a mode bounce issue which occurs at the transition of two operation modes. If the mode bounce occurs, the converter goes back and forth between two operation modes and looks unstable as shown in FIG. 1 prior art. FIG. 1 prior art shows from top down a signal at LX1 node, a signal at LX2 node and an inductor current. The switching nodes LX1 and LX2 are located at a first and a second terminal of the inductor as shown in FIG. 5. As a result, starting with an operation mode change, the output voltage suffers from significant ripples and the efficiency is degraded. To avoid the mode bounce, there needs a large offset voltage on the error amplifier output as shown in FIG. 3a prior art when the operation mode changes and it causes a large output ripple during the transition. FIG. 2 prior art shows from top down the signal at LX1 node, the signal at LX2 node, the output voltage, and an inductor current when a mode change occurs.
FIG. 5 prior art shows a prior art buck-boost switching configuration having 4 switches S1-S4. According to the switching state, this can be a buck topology having two switches S1-S2 as shown in FIG. 6a prior art or a boost topology having two switches S3-S4 as shown in FIG. 6b prior art.
It is a challenge for the designers of buck-boost switching regulators to suppress operation mode bounce and minimize ripples/glitch when an operation mode change is required.